The 40,000 Russian troops massed close to the border with Ukraine could attack with little warning and seize control of parts of the country in a matter of days, the top NATO commander said in a new interview.

General Philip M. Breedlove warned in an interview with the New York Times that the potent mix of warplanes, helicopter units, artillery, infantry and commandos could move with just 12 hours notice and achieve Russia’s military objectives just a few days.

“Essentially, the force is ready to go,” Breedlove said. “We believe it could accomplish its objective between three to five days. I think they have all the opportunities and they can make whatever decision they want.” Reflecting skepticism that the troops are massed for a training exercise like Russian officials have said, Breedlove added: “This is a very large, very well-equipped force to be called an exercise.”

Breedlove’s comments suggest the recent movement of Russian troops away from the border was part of a regular rotation, rather than the pullback some had hoped.

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“What we can say now is that we do see a battalion-size unit moving, but what we can’t confirm is that it is leaving the battlefield,” Breedlove said. “Whether that movement is aft to a less belligerent configuration or returning to barracks, we do not see that.”